A predistortion linearizer employing digital signal processing (hereinafter referred to as a digital predistortion method) has been known as one of non-linear distortion compensation methods of a microwave power amplifier (for example, Patent literature 1). The digital predistortion method is characterized by eliminating a complicate analog circuit by enabling a predistorter to be configured with digital signal processing.
As a configuration of a digital predistorter, a configuration with a lookup table having a table for previously linearizing non-linear characteristics of an amplifier has been known (for example, Non Patent literature 1 and Patent literature 2). The digital predistorter with a lookup table feeds back an output signal of the amplifier and updates the setting values of the lookup table so as to make a distortion component equal to or less than the designed value. In this manner, the digital predistorter is known as being able to compensate for distortion components.
A predistorter based on a power series model has been known. The power series type predistorter has been realized by an analog circuit, achieving the distortion improvement equal to or more than 30 dB (Non Patent literature 2). The power series model has been known to accurately model non-linear characteristics of an amplifier (for example, Non Patent literature 3). In a distortion compensation method in a digital predistorter using a power series model, a signal for correcting a coefficient of each order needs to be extracted from an output signal of the amplifier by the power series model. The technique disclosed in Patent literature 1 extracts a correction signal by removing a fundamental wave from an output signal of a power amplifier. As a method for more simply extracting a correction signal of a power series model, a method using two carrier waves of different frequencies and an equal level as a pilot signal has been known (Non Patent literature 2).
For the existing wireless systems, for example, a PDC (Personal Digital Cellular), a GSM (Global System for Mobile Communications), and an IMT-2000 (International Mobile Telecommunication 2000) have been operated. On the other hand, there is a technique for implementing a radio apparatus by software so that the radio apparatus can accommodate a plurality of wireless systems by using a single hardware. If a radio apparatus can accommodate a plurality of wireless systems by using a single hardware, a user of a single hardware can use a single mobile communication provided by a wireless system without thinking about the wireless system or a core network in the background of the wireless system. Actually, however, a single hardware accommodating a plurality of wireless systems has not been realized yet.
It may become that the services to be provided by wireless systems would be different for each area or each operator, while the wireless systems would be diversified. As a result, wireless systems optimum for respective objects may need to coexist in the same time and at the same place.
As a method using the plurality of wireless systems, a multi-band wireless system has been known. The multi-band wireless system adaptively varies a frequency band to use or the number of frequency bands to use according to a propagation environment or a traffic state. In order to reserve a predetermined transmission quality or a predetermined transmission capacity, a multi-band transmission using unused frequency bands is effective. In the multi-band wireless system, the number of frequency bands to be used is always varied according to the transmission state which should be guaranteed in each wireless system. The number of channels to be used also varies in the same band. If a frequency band used by a business entity and a frequency band used by anther business entity coexist, the multi-band wireless system can improve efficiency in using a frequency by executing adaptive control on the use of a free frequency band through technologies of interference recognition, frequency sharing, interference canceling, reduction in giving-interference, and multi-band controlling.
In a power series type predistorter for a base station adapted to such a multi-band wireless system, there has been a method for composing a power series type predistorter that has distortion generators corresponding to a plurality of frequency bands and sharing a conventional delay line of a power series type predistorter. In a method for arranging such a plurality of power series type predistorters merely in parallel, transmitting signals in a plurality of frequency bands are inputted into respective distortion generators. Each distortion generator adjusts an amplitude and a phase of each inputted transmitting signal so that distortion compensation is executed for each frequency band.
If transmitting signals to be inputted into the distortion generators means are transmitting signals in a plurality of frequency bands, optimum adjustment of amplitudes and phases cannot be executed on the transmitting signals in respective frequency bands. For example, if the transmitting signals are in 800 MHz band and in 1.5 GHz band, the distortion generator can set optimum amplitude and phase for 800 MHz band but requires amplitude and phase setting means which enables high-speed operation for following the difference in frequency by 700 MHz to set optimum amplitude and phase in 1.5 GHz that is 700 MHz different from 800 MHz. However, such a high-speed amplitude and phase setting means has not been developed yet.
Even with a plurality of power series type predistorters that operate in respective frequency bands, a power series type predistorter that operates in a plurality of frequency bands cannot be created.
In a multi-band wireless system with a plurality of transmitting bands, frequency bands can be changed according to a service state of the wireless system, interference to another wireless system, or the like. When a frequency band or the like for a wireless system is changed, a conventional power series type predistorter that fixes a frequency band to compensate for distortion cannot adaptively change operating frequency.
A power series type predistorter to be used in a long time needs to execute improvement and change on a power series type predistorter at each base station, thus needs tremendous efforts and time in readjusting so many power series type predistorters to change frequency bands. A power series type predistorter configuration that can be economized by eliminating such efforts and time is required.
For example, a power series type predistorter, which enables distortion compensation in the frequency bands f1 and f2 simultaneously, cannot execute the distortion compensation in the frequency bands f1 and f3 simultaneously when the frequency band f2 is changed to f3. This is because that loop adjustment cannot be done due to the fixed operating band of a conventional power series type predistorter and a difference between frequencies of f1 and f3 as mentioned above.    Patent literature 1: UK Patent Publication No. 2,335,812    Patent literature 2: National Publication of International Patent Application No. 2002-522989    Non Patent literature 1: H. Girard and K. Feher, “A new baseband linearizer for more efficient utilization of earth station amplifiers used for QPSK transmission”, IEEE J. Select. Areas Commun. SAC-1, No. 1, 1983.    Non Patent literature 2: T. Nojima and T. Knno, “Cuber predistortion linearizer for relay equipment in 800 MHz band land mobile telephone system”, IEEE Trans. Vech. Tech., Vol. VT-34, No. 4, pp. 169-177, 1985. 11.    Non Patent literature 3: Tri T. Ha, Solid-State Microwave amplifier Design, Chapter 6, Krieger Publishing Company, 1991.